Mobility has been a key research and business issue in the last decades. Many layer 2 wireless access systems have evolved like WLAN (WiFi) or WiMax. These systems solve the handover problem of a terminal moving into the range of another base station (wireless access point). The question of mobility in layer 3 (i.e. IP layer) is how to retain the IP address of a terminal that re-attaches at another router or performs handover to another router. Ideally, the handover process should be seamless from the user's point of view.
Mobile IP (MIP) is a technology to enable mobility in the IP layer. The mobile node (MN) always retains its home address (HoA). Traffic destined to the home address of the terminal is tunneled to the new location of the mobile from the home agent (HA) that resides in the home network of the MN. Since the home address of the terminal is topologically incorrect in visited networks, tunneling is needed for transport. The home agent uses the new local address of the terminal as the destination of the tunnel; hence the mobile node needs to keep the home agent informed about its new location information by sending binding update (BU) messages. These messages periodically notify the home agent about the new local address (also known as care-of-address, CoA) of the terminal. The home agent uses a binding cache to store the current CoA of the mobile node. The mobile node is required to be configured with some parameters relating to MIP. It must know its own home address, the IP address of its home agent and also needs a mechanism to acquire IP address on a visited network (DHCP or Neighbour Discovery).
In other words, Mobile IP (e.g. MIPv4, MIPv6) allows a mobile node to change its point of attachment to the Internet with minimal service disruption. For example, the MIP version 6 (MIPv6) protocol [1] allows nodes to move within the Internet topology while maintaining reachability and on-going connections with correspondent nodes.
MIPv6 capable mobile nodes, such as cellular phones, laptops and other end-user equipment, can thus roam between networks that belong to their home service provider as well as others. Roaming in foreign networks is enabled as a result of the service level and roaming agreements that exist between operators. MIPv6 provides session continuity within a single administrative domain, but typically depends on the availability of an Authentication, Authorization and Accounting (AAA) infrastructure to provide services across different administrative domains, i.e. when roaming outside the network administered by the home operator.
For a better handover performance some major improvements have been proposed. A key observation is that mobile handover in most cases happens between neighbouring base stations. In this case the mobility should be handled somehow “locally” since traffic can still arrive at the same root gateway of the network. This idea led to create a hierarchy, and split mobility into micro- and macro-mobility.
Micro-mobility handles the movements within a geographically limited area, called mobility domain. Mobility domains are usually under the management of a single entity resulting in a protected and trusted environment between network nodes. Inside mobility domains mobility can be handled locally, which makes it fast and transparent from the perspective of network areas outside the mobility domain.
Mobility between domains is handled by macro-mobility mechanisms, which do not have to be quick, since such mobility is rare. On the other hand, macro mobility must operate globally in a less trusted environment. Examples of micro-mobility protocols include Cellular IP and Hawaii, while Mobile IP is the prime example of IP layer macro-mobility.
Hierarchical Mobile IPv6 (HMIPv6) [2] is a micro-mobility protocol re-using MIPv6 messages and mechanisms. As illustrated in FIG. 1, HMIPv6 adds a new hierarchy level to existing Mobile IPv6 architecture by adding a new entity called Mobility Anchor Point (MAP). Mobility under the umbrella of a MAP 1-1, 1-2 is handled by HMIP, whereas mobility between MAPs is handled by MIP. The MAP can help in providing seamless mobility for a moving mobile node 2-2, while the mobile node is communicating with a correspondent node (CN) 3. As previously explained, the mobile node (MN) always retains its home address (HoA). Traffic destined to the home address of the mobile node is tunneled to the new location of the mobile from the home agent (HA) 4 that resides in the home network of the MN. In HMIPv6 mobile terminals in the visited network have two types of care-of-address: one for current link, called On-Link Care-of-Address (LCoA), and another that is allocated at the current MAP 1-2, called Regional Care-of-Address (RCoA). The binding between these care-of-addresses is kept by the MAP 1-2, which acts like a “local home agent” for the mobile node 2-2. In case of intra-area handover the mobile node or terminal 2-2 sends binding update (BU) messages for the LCoA-RCoA binding to the MAP 1-2. If the mobile node moves to another area, then the MN sends regular MIP binding update messages for the new RCoA-HoA binding to the Home Agent 4.
All previous solutions rely on extensive support from the terminal. Most importantly, a mobility (e.g. Mobile IP) protocol stack has to be implemented in the terminal. However, as terminals typically do not implement Mobile IP today, such solutions are impractical for everyday laptops and PDAs running, e.g., different versions of the Microsoft Windows operating system. Therefore, another mobility research direction tries to create an architecture where the terminal is not expected to perform new functions but the required functions are moved to the network. This is called network-based or network-centric mobility, where the complexities assigned to micro-mobility and fast handover are placed in the network. The intention is to be able to move and make seamless handovers across routers without adding mobility specific functions to terminals.
For example, the NETLMM working group of IETF is working on network centric mobility protocols. Several approaches have already been investigated, one proposed by a design team [5] and another called Proxy-Mobile IP (PMIP) [6]. The solution proposed in reference [5] defines a separate protocol to manage mobility between a Local Mobility Agent (LMA) and Mobility Access Gateways (MAG). MNs attach to one of the MAGs. The LMA stores the identity of current MAG and tunnels downlink packets to it. The PMIP solution employs a Home Agent as anchor and uses MIP signaling for mobility update. However, the functions that reside in the MN in case of MIP are moved to the access router, thereby eliminating the need for mobility specific functions in the Mobile Node.
Each of the above technologies is unfavorable due to at least one of the following reasons:                requires new functionality in the terminal, and        requires many new functions in network nodes.        
Mobile IP, HMIP, Cellular IP or Hawaii require the implementation of the corresponding protocols in the terminal. As a matter of fact, Microsoft, for example, has no plans to include IP mobility (more specifically Mobile IP) support in the upcoming release of its operating system.
PMIP and other network-centric protocols, on the other hand, do not require support from the terminal; however, they require significant amount of new functions (e.g., the implementation of the new protocol) from the network nodes, which are not available in current IP routers.